The present invention relates to a method and apparatus for mixing various flows into a process liquid flow. The present invention is suitable for use in processing process liquids of all industrial branches. Introducing various chemicals into the stocks, stock components and fibrous suspensions of paper and pulp industry can be mentioned as an especially preferable application for the method and apparatus according to the invention.
In the following description, embodiments of the present invention are described in the context of papermaking. This must, however, be understood only as one example of the various applications of the invention, because similar applications for mixers, problems with mixing and desire to solve them can be found at a wide variety of industrial branches. In papermaking, similar to countless other branches of industry, there are needs for mixing a substance, hereinafter called a chemical in the widest possible meaning of the term, whereby the term covers plain water (more generally a liquid), air (more generally a gas or steam) as well as introducing some other solid material, not excluding various treatment chemicals and other chemicals, into a pipe flow. In some cases it is enough to let the desired amount of chemical to flow into a pipe flow so that it is mixed with the flowing material, a liquid or a gas, by the turbulence in the actual pipe flow. Sometimes the desired amount of the chemical is drained into such a point of a pipe flow where there is a turbulence-producing mechanical apparatus slightly after the chemical addition point, either a static flow hindrance, a rotary mixer or, for example, a centrifugal pump. In some cases the chemical is introduced into a relatively large tank arranged in the process, either directly or, for example, with a substance directed into the tank, whereby the necessary mixer is arranged in the tank.
In many cases there however is a need for a considerably faster and more efficient method of mixing. An example of such could be, e g mixing a chemical with very fast reactions, such as ozone, into cellulosic fiber suspension. If the mixing is carried out slowly, ozone has time to spoil the part of the pulp located close to the chemical introduction opening while a part of the pulp remains totally untreated, because the ozone does not have the time to reach said portion of the pulp, but it is instead used up earlier. Such a chemical needs a very fast and complete method of mixing.
Another example could be, e.g. introducing into the stock two such chemicals that are supposed to react with each other and to form filler particles of even size or to form, for example, micro flocks with the fibers or the fine material of the stock. If slow mixing methods are used in such applications, it is obvious that there are, e.g. the following kinds of problems:
(i) The size of the particles varies within a wide range because the whole time when both chemicals are present in the stock both new particles are formed and the size of the old particles is increased.
(ii) This also applies to the formed flocks, the size of the flocks varies for exactly the same reason.
(iii) Further, as the purpose is to fasten the fine material of the stock to the fibrous material by means of retention chemicals, they must be introduced in such an amount that there surely is enough for all places of the stock flow despite the long duration of the mixing.
The above-mentioned problems are also discussed in patent documents EPB11064427, EP-B1-1219344, FI-B-111868, FI-B-115148 and FI-B-116473 of Wetend Technologies Oy, in which injection mixing using injection liquid is presented as a solution for fast mixing. Suitably arranging the injection nozzles to the circumference of the process pipe so that one mixer is sufficient for pipes of small diameter, slightly larger pipes use two opposing nozzles on the same circumference, pipes slightly larger than this need three nozzles located at 120 degree intervals on the circumference and so on, provides the currently operationally best mixing arrangement for e.g. introduction of the retention chemicals of papermaking and corresponding mixing.
As there is in some applications a need to introduce a number of chemicals essentially simultaneously, document FI-B-116473 discloses an introduction arrangement in which in the nearhood of the injection nozzle discussed in the above-mentioned patents there is, directly upstream thereof, an opening wherefrom a second chemical is allowed to flow in desired amounts to the flow/process pipe with a just sufficient pressure difference so that the second chemical flows along the inner surface of the process pipe to the opening of the injection nozzle, wherefrom the fast jet of injection liquid and the second chemical entrains and mixes the second chemical as well into the process liquid.
However, the following problems, among others, have been found in the above-mentioned solutions:
(i) in most demanding conditions the mixing is not as efficient and fast as desirable.
(ii) one injection jet is not sufficient for mixing a very large amount of a second chemical.
(iii) in some cases there has been a need for a relatively long distance between the introduction points of the two chemicals, i.e. of the order of >2 seconds, for the first chemical to be mixed evenly enough into the whole flow. In practice, in the short circulation of a paper machine, for example, this means a distance of over five meters between two mixers.
It is worth mentioning, as a separate problem from the previous problems, the tendency of some chemicals or their reaction products to precipitate on or fasten to the surfaces of all solid materials. Thus there can be, in addition to the desirable precipitation on the surfaces of the fibers of the stock or other solids in the suspension, also precipitation on or fastening to the surfaces of the actual process pipe or the structures located therein (including the various surfaces of the mixer). Such a precipitation or fastening is by no means desirable, as at some point the precipitation or particles/pieces detaching therefrom will in some way be detrimental to the production of the final product or even detrimental to the quality of the final product.